Tension member fatigue tester using transverse resonance

ABSTRACT

A system includes: a tension member having a first end and a second end, where the first end of the tension member is connected to a first loading member and the second end of the tension member is connected to a second loading member; a first actuator configured to translate the first loading member, such that a tensile load is applied to the tension member along a first direction; a second actuator configured to translate the second loading member in two or more second directions that are substantially transverse to the first direction; and a control system that is configured to control the second actuator, such that the second loading member oscillates between the two or more second directions, where the oscillation of the second loading member causes the tension member to vibrate at a frequency.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. ProvisionalApplication No. 61/867,917, filed Aug. 20, 2013 and U.S. patentapplication Ser. No. 14/066,667, filed Oct. 29, 2013. The entire contentof the aforementioned applications are herewith incorporated byreference into the present application.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Tension members, such as tethers, umbilicals, ropes, and cables, areused to support loads associated with overhead transmission lines,suspension bridges, aerostats, offshore drilling platforms, and towingand mining applications, among other uses. In many such applications,the tension members undergo repeated loading, often at very high loadvalues and over a large number of cycles. Thus, a testing system isuseful to repeatedly test tension members under anticipated loadconditions. Information from such tests may help to determine, forexample, how well a tension member product or design will hold up infatigue.

SUMMARY

Various tension member test systems and methods for the fatigue testingof tension members are disclosed herein. Specifically, a tension memberunder tension could be tested by displacing one end of the tensionmember back and forth in at least one direction substantially transverseto the long axis of the tension member. Such displacements may inducethe tension member to vibrate at a frequency. The frequency could relateto a resonant frequency of the loading system.

In a first aspect, a system is provided. The system includes a tensionmember having a first end and a second end. The first end of the tensionmember is connected to a first loading member and the second end of thetension member is connected to a second loading member. The system alsoincludes a load cell, a first actuator, a second actuator and a controlsystem having a processor and a memory. The processor is configured toexecute instructions stored in the memory so as to carry out operations.The operations include controlling the first actuator to displace thefirst loading member based on information received from the load cellsuch that a tensile load is applied to the tension member along a firstdirection. The operations also include controlling the second actuatorto displace the second loading member based on a desired cyclic loadingprofile such that the second loading member oscillates between two ormore second directions. The oscillation of the second loading membercauses the tension member to vibrate at a frequency. The desired cyclicloading profile includes a base load and a peak load.

In a second aspect, a method is provided. The method includescontrolling a first actuator to displace a first loading member based oninformation received from a load cell such that a tensile load isapplied to a tension member along a first direction. The tension memberhas a first end and a second end. The first end of the tension member isconnected to the first loading member and the second end of the tensionmember is connected to a second loading member. The method also includescontrolling a second actuator to displace the second loading memberbased on a desired cyclic loading profile such that the second loadingmember oscillates between two or more second directions. The oscillationof the second loading member causes the tension member to vibrate at afrequency. The desired cyclic loading profile includes a base load and apeak load.

In further aspects, any type of device could be used or configured as ameans for performing functions of any of the methods described herein(or any portions of the methods described herein). For example, variousmechanical apparatuses could be used to apply a base tension load to atension member using any type of a first loading member along a firstdirection. Furthermore, any type of a second loading member coulddisplace the tension member in a direction substantially transverse tothe first direction so as to vibrate the tension member at a frequency.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a transverse resonance fatigue testsystem, according to an illustrative embodiment.

FIG. 2A is a diagram illustrating details of the transverse resonancefatigue test system, according to an illustrative embodiment.

FIG. 2B is a diagram illustrating details of the transverse resonancefatigue test system with multiple tension members, according to anillustrative embodiment.

FIG. 2C is a diagram illustrating details of the transverse resonancefatigue test system with a flexible element, according to anillustrative embodiment.

FIG. 3 is a simplified block diagram illustrating various components ofthe transverse resonance fatigue test system, according to anillustrative embodiment.

FIG. 4 is a simplified block diagram illustrating a method, according toan illustrative embodiment.

DETAILED DESCRIPTION

Illustrative methods and systems are described herein. The illustrativeembodiments described herein are not meant to be limiting. It will bereadily understood that certain aspects of the disclosed systems andmethods can be arranged and combined in a wide variety of differentconfigurations, all of which are contemplated herein.

Furthermore, the particular arrangements shown in the Figures should notbe viewed as limiting. It should be understood that other embodimentsmay include more or less of each element shown in a given Figure.Further, some of the illustrated elements may be combined or omitted.Yet further, an illustrative embodiment may include elements that arenot illustrated in the Figures.

I. Overview

In an Airborne Wind Turbine (AWT) system, an aerial vehicle may convertkinetic wind energy to electrical energy. The aerial vehicle may beconnected to a ground station via a tether so that (i) the aerialvehicle can take off from the ground station, fly at a range ofelevations (for example, in a substantially circular path), land at theground station, and (ii) conduct electrical current to the groundstation. In some situations, the ground station may transmit electricityto the aerial vehicle (for example, during take-off, landing, and/or lowwind conditions). The tether could also be configured to transmit otherkinds of signals.

Illustrative embodiments may generally relate to or take the form ofsystems and methods for testing tethers that can be used in AWTs. Moregenerally, such testing systems and methods may be utilized to test“tension members.” Tension members may include, but are not limited to,tethers (and associated connection hardware) utilized in a wind energysystem such as an AWT, as well as other types of cables or lines.

Such testing systems and methods could cause a tension member to vibratein a transverse fashion while under tension, similar to a plucked guitarstring. In some embodiments, systems and methods could cause the tensionmember to vibrate at or near a resonant frequency. The resonantfrequency could relate to a resonance arising due to a combination ofloading system elements and/or the environment.

The tension member could vibrate in a transverse mode or transversedirection. A transverse mode could be, for example, a vibration of thetension member where the vibration has an amplitude perpendicular, ortransverse, to the long axis of the tension member. By causing thetension member to vibrate in such a manner, the tension force on thetension member may vary in a periodic fashion. Tension members mayinclude, but are not limited to, tethers (and associated connectionhardware) utilized in a wind energy system such as an AWT.

In an illustrative AWT system, a tensile load on the tether (and, insome examples, its connections to the aerial vehicle) may vary. Forexample, the tensile load on the tether may vary based on aerodynamicand gravitational forces acting on the aerial vehicle and/or one or moreturbines connected to the aerial vehicle. Specifically, the aerodynamicforces acting on the aerial vehicle may vary based on factors such asthe net thrust or drag of the turbines, control surface configuration,altitude, relative wind speed/direction, and/or flight speed of theaerial vehicle.

Additionally, variations in tensile load on the tether could occur in aperiodic manner, such as during circular flight. In particular, the loadon the tether could vary periodically during normal “cross-wind” flightwhere the aerial vehicle flies in a substantially circular flight path.In some embodiments, a time period of a single circular flight cycle,and thus the periodic variation of tensile load on the tether, could bebetween 14 to 19 seconds. Further, in some embodiments, the tensile loadon the tether may vary by about 50% during the circular flight cycle ofthe aerial vehicle.

In the foregoing example, the tether may experience more than twomillion cycles of tensile loading between 100,000 and 220,000 Newtons(N) per year due to circular flight cycles alone. Further, the tensileload on the tether may vary in other ways as well. For example, thetensile load on the tether may vary when the ground station retrievesthe aerial vehicle by reeling in the tether. Such repeated variations intensile loading may cause the tether to fail (e.g., rupture, tear,crack, rip, or otherwise not function as part of an AWT).

Embodiments described herein may be used to test one or more parametersof a tether, such as fatigue performance, stiffness, maximum static loadcapability, and/or strength, by loading the tether before or after it isused as part of an AWT system.

Beneficially, the embodiments described herein may be used to testfatigue performance of a tether in a time period that is shorter than adesired lifespan of an AWT tether. For instance, in someimplementations, fatigue performance of a tether with a desired life oftwo years as part of an AWT may be tested in four days. As a result, thetime that is needed to design and/or evaluate new tether designs may bereduced. Further, embodiments may help to achieve such time savingswhile at the same time reducing the cost of such testing by, e.g.,reducing the size and/or complexity of testing components. Finally, dueto such time savings, more tests may be performed in a given period oftime, which could improve the confidence level of data gathered fromsuch systems and methods. Accordingly, the system and methods disclosedhere could provide better ways to help: 1) evaluate and compareperformance of different tether and termination designs; 2) uncoverflaws in manufacturing or design of such tethers earlier in the testingprocess; and 3) build confidence in tether/termination design andoverall system design.

II. Illustrative Systems

FIG. 1 is a simplified diagram illustrating a testing system 100according to an illustrative embodiment. As shown, the testing system100 includes a first loading member 104, a first actuator 108, a secondloading member 106, a second actuator 110, and a control system 120.Further, as shown in FIG. 1, a tension member 102 has a first end thatis connected to the first loading member 104 and a second end that isconnected to the second loading member 106.

In some implementations, the first actuator 108 may be configured todisplace the first loading member 104, such that a tensile load (e.g.,an initial base load) is applied to the tension member 102 along a firstdirection 112. In some examples, the first actuator 108 may displace thefirst loading member 104 by applying one or more forces to the firstloading member 104.

Moreover, in some implementations, the second actuator 110 may beconfigured to displace the second loading member 106 back and forth intwo or more second directions 114, such that the two or more seconddirections 114 are substantially transverse to the first direction 112.Such displacement could be likened to that of wiggling a free end of ataut string, the other end of which is fastened to a wall. In an exampleimplementation, the second actuator 110 could have a stroke length ofthree inches, although other implementations are possible.

The term “substantially transverse,” as used in this disclosure, refersto exactly transverse and/or one or more deviations from exactlytransverse that do not significantly impact testing a tension member asdescribed herein.

Further, in some implementations, the control system 120 may beconfigured to control the second actuator 110, such that thedisplacement of the second loading member 106 causes the tension member102 to vibrate at a particular frequency, such as a resonant frequencyof a combination of loading system elements that could include thetension member 102. The resonant frequency may vary based, at least inpart, on a change in the stiffness of the loading system. Such a changein stiffness may be due to an amount of deflection of the tension member102 and/or the second loading member 106. As a result, the controlsystem 120 may control the second actuator 110 to (i) cause vibration ofthe loading system at its resonant frequency; and (ii) maintain theamplitude of vibration by controlling the amount of energy gained orlost by the loading system (for example by controlling the magnitude orduty cycle of the input force from the second actuator 110).

For example, the control system 120 could be configured control thesecond actuator 110 so as to cause the tension member 102 to vibrate ina transverse mode. In some examples, the control system 120 may beconfigured to control the second actuator 110 based on at least oneangular position of the second loading member 106 during thedisplacement cycle.

Further still, in some implementations, repeated displacement cycles ofthe second loading member 106 may cause the tension member 102 to beperiodically loaded substantially in tension. Beneficially, such loading(rather than, for example, directly pulling a middle portion of thetension member 102) may reduce local compressive stress in the tensionmember 102 that could affect testing of one or more parameters of thetension member 102. Moreover, such an illustrative configuration maybeneficially permit testing of fatigue performance of tension members athigh frequencies, which may increase the speed of evaluating and/ordesigning such tension members.

The term “substantially in,” as used in this disclosure, refers toexactly in and/or one or more deviations from exactly in that do notsignificantly impact testing a tension member as described herein.

Additionally, as shown in FIG. 1, the system 100 may further include aframe 116. Further, as shown in FIG. 1, the first loading member 104 maybe connected to the tension member 102 via a first connection locationand the first loading member 104 may connected to the frame via a secondconnection location; and the second loading member 106 may be connectedto the tension member 102 via a first connection location and the secondload member 106 could be connected to the frame 116 via a secondconnection location. In some embodiments, the second loading member 106could be rotatably connected to the frame 116 and/or the second end ofthe tension member 102 via cylindrical roller bearings (not shown).

In some implementations, the frame 116 may be configured to resist oneor more tensile forces and/or compressive forces based on thedisplacement of the first loading member 104 and the displacement of thesecond loading member 106. The frame 116 could include, for example, afour-member compression frame with lacings and battens. In someembodiments, the frame 116 could be made of structural steel. However,the frame 116 could include different structural elements or materials.

As shown in FIG. 1, the frame 116 (and correspondingly, the firstdirection 112) could be oriented substantially parallel to the ground118. In such embodiments, the displacement of the second loading member106 may be substantially perpendicular to the ground 118 (e.g., thesecond loading member could be configured to move up and down relativeto the ground 118). Alternatively, the displacement of the secondloading member 106 may be substantially parallel to the ground 118(e.g., side-to-side with respect to the first direction 112). In suchimplementations, when the displacement of the second loading member 106is substantially parallel to the ground 118, the system might includecables and/or beams configured to reduce at least one out-of-planeand/or rotational mode of vibration of the tension member 102 during thedisplacement of the second loading member 106. In other words, the longaxis of tension member 102 could be substantially parallel orperpendicular to a gravitational force vector 122. Accordingly, based atleast in part to the orientation of the frame 116, the vibration oftension member 102 could be affected by gravitational force duringvibration.

In other examples, the frame 116 may be oriented such that the firstdirection 112 is substantially perpendicular to the ground 118. In suchexamples, the displacement of the second loading member 106 may besubstantially parallel to the ground. In other examples, the firstloading member 104 and/or the second loading member 106 may be connectedto the ground or any other type of fixture. In such examples, the systemmight not include a frame 116.

The term “substantially parallel,” as used in this disclosure, refers toexactly parallel and/or one or more deviations from exactly parallelthat do not significantly impact testing a tension member as describedherein. Further, the term “substantially perpendicular,” as used in thisdisclosure, refers to exactly perpendicular and/or one or moredeviations from exactly perpendicular that do not significantly impacttesting a tension member as described herein.

Although FIG. 1 shows one tension member 102 connected to the firstloading member 104 and the second loading member 106, in other examples,two or more tension members 102 may be connected to the first loadingmember 104 and the second loading member 106.

FIG. 2A is a simplified diagram illustrating selected details of thetesting system 200. The system may further include one or moretermination assemblies 202 & 204, a coupler 208, a first sensor 206, anda second sensor 210. Specifically, the first and/or second ends of thetension member 102 could include termination assemblies 202 & 204.Termination assemblies 202 & 204 could include potted terminationfixtures, although other types of termination assemblies are possible.As shown in FIGS. 1 and 2A, the first end of the tension member 102 isconnected to the termination assembly 202. Thus, in some embodiments,the first end of the tension member is connected to the first loadingmember 206 via the termination assembly 202. In some implementations,the testing system could be configured to test at least the combinationof the termination assemblies 202 & 204 and the tension member 102.

Further, as shown in FIG. 2A, the second end of the tension member isconnected to the coupler 208 via termination assembly 204. In someembodiments, the second end of the tension member 102 is connected tothe second loading member 106 via the coupler 208. In some examples, thecoupler 208 may be further configured to receive one or more weights(not shown). In some examples, various elements of the system 200 couldbe likened to a harmonic oscillator. In such a situation, the tensionmember 102 and the weight applied to the coupler 208 could be treatedlike the spring and mass in such a harmonic oscillator. By displacingthe tension member 102, the system could act similar to a drivenharmonic oscillator system. As such, adding one or more weights to thecoupler 208 could affect the testing of the tension member 102 (e.g., bymodifying a resonant frequency of the loading system, which acts as aharmonic oscillator system).

Additionally, as shown in FIG. 2A, the first sensor 206 could include aload cell (not shown) that senses stress (or force) information. Forexample, the first end of the tension member 102 could be connected tothe first loading member 104 via the first sensor 206. In such ascenario, the first sensor 206 could be configured to sense the tensionforce on the tension member 102. Thus, the first sensor 206 couldmeasure tension forces before, during, and/or after load testing.Further, the first sensor 206 could be used to calibrate the systembefore, during, and/or after load testing. The first sensor 206 maytransmit force information to the control system 120 on a continuousbasis, a periodic basis, or in response to a prompt from the controlsystem 120. In some embodiments, tension information from the firstsensor 206 could be used to “pre-load” the tension member 102 with abase tension force prior to beginning transverse mode testing. Asdiscussed below, the control system 120 may be configured to control thefirst actuator 108 based, at least in part, on data received from thefirst sensor 206. Additionally or alternatively, the control system 120may be configured to control the second actuator 110 based, at least inpart, on data received from the first sensor 206.

In some embodiments, the second sensor 210 could be configured tomeasure the displacement, angle, or rotation of the second loadingmember 106 with respect to a reference angle or reference point. Thereference angle could relate to an angle based on the current positionof the tension member 102, the second loading member 106 or the coupler208. Other reference angles are possible. The reference point could be apoint along an axis oriented in the first direction 112 or any otherpoint associated with the position of the tension member 102, coupler208, the second loading member 106, or the second actuator 110. Otherreference points are possible. The second sensor 210 could be located onor near the secondary loading member 106, the coupler 208, the secondactuator 110, or the frame 116. Other locations for the second sensor210 are possible. The second sensor 210 could include an optical encoderor a magnetic encoder operable to sense position and/or orientation ofsystem components. For example, a magnetic linear encoder could bemounted near a shaft of the second actuator 110. The magnetic linearencoder could be configured to sense a position with respect to amagnetic reference attached to the shaft of the second actuator 110. Insome implementations, the second sensor 210 may be a rotary encoder thatsenses angular position information of the second loading member 106before, during, and/or after displacement of the second loading member106. Other configurations for the second sensor 210 are possible. Forexample, the second sensor 210 could be configured to measure thedisplacement or angle of the tension member 102 or the coupler 208.Alternatively, the second sensor 210 could be an optical image sensor ora camera. In some embodiments, the second sensor 210 could be configuredto transmit angular and/or positional information on a continuous basisto the control system 120. In other embodiments, the second sensor 210could be configured to transmit angular and/or positional information tothe control system 120 on a periodic basis, or to transmit such data inresponse to a signal from the control system 120. As discussed below,the control system 120 could be configured to control the secondactuator 110 based, at least in part, on the data received from thesecond sensor 210.

Some implementations could include a third sensor (not shown), such as apressure transducer. In such implementations, the third sensor may beconfigured to sense a pressure of a hydraulic or pneumatic cylinder. Thethird sensor could be located, for instance, on a tee at the cylinderend port of the first actuator 108. The third sensor could be configuredto sense pressure data for use by the control system 120. Such pressuredata could be used by the control system 120, for example, to determinethe tensile load on the tension member 102.

The control system 120 could utilize information from the first sensor206, the second sensor 210, and/or the third sensor to establish a cycleload profile. The cycle load profile could include information about thetensile load on the tension member 102 throughout one or more loadingcycles (base load, peak load, cycle period, etc.).

FIG. 2B is a simplified diagram illustrating selected details of amultiple tension member testing system 220. In some embodiments, such asystem could be configured to test two or more tension memberssimultaneously or at different times. For example, a first tensionmember 224 could be connected to a first loading member 222 and acoupler 226. The coupler 226 could be connected to an actuator 228. Asecond tension member 230 could also be connected to the coupler 226.The other end of the second tension member 230 could be connected to asecond loading member 232. The first loading member 222 and the secondloading member 232 could be configured to move in a directionsubstantially along the long axis of the tension members. Alternatively,the second loading member 232 need not be configured to move. That is,the second loading member 232 could be a fixed connection point for thesecond tension member 230. The actuator 228 could be configured to movein two or more directions substantially transverse to the long axis ofthe tension members. In other words, a single actuator could beconfigured to move back and forth in a direction transverse to one ormore axial directions so as to test multiple tension memberssimultaneously. Although two tension members are shown in FIG. 2B, morethan two tension members could be tested in such an embodiment. Forexample, multiple tension members could be loaded in parallel betweenthe first loading member 222 and the second loading member 232. Yetfurther, in reference to FIG. 1, the second loading member 106 could bereplaced with a second tension member. Other arrangements of tensionmembers and loading members are possible.

FIG. 2C is a simplified diagram illustrating selected details of atension member testing system 240 with a flexible element. In an effortto modify, adjust, or optimize the resonant frequency of the loadingsystem, embodiments may further include one or more flexible elements,such as a spring. For instance, as shown in FIG. 2C, the system mayfurther include a flexible element 248 located between the frame 252 andthe second loading member 250, located between the frame 252 and thecoupler 244, and/or located between the frame 252 and the secondactuator 246. And, as shown in FIG. 2C, the flexible element 248 may belocated between the coupler 244 and the second actuator 246.

In such implementations, the flexible element 248 may compensate for agravitational force on the coupler 244 and/or the second loading member250 during the oscillation of the tension member testing system 240. Invarious embodiments, the flexible element 248 could include an in-linespring (e.g., connected between the second loading member 250 and thetension member 242), transverse springs (e.g., connected between thetension member 242 and the frame 252), and, in some embodiments, tunedmass dampers. Accordingly, in an illustrative embodiment, a spring couldbe connected between the coupler and the load frame in order tocounteract gravity's effect of differences in peak load at the lower andupper extremes of tension member movement. Although one flexible element248 is shown in FIG. 2C, multiple flexible elements are possible and theposition of the one or more flexible elements could vary. For example,the flexible element 248 could be connected to or incorporated into thecoupler 244. Other positions for the flexible element 248 are possible.

FIG. 3 is a simplified block diagram of selected elements of a tensionmember testing system. A control system 300 could include amicrocontroller 302, memory 304, instructions 306, and acomputer-readable medium 308. The system could include one or moresensors 320, including a temperature/humidity sensor 322, an encoder324, a pressure transducer 326, and a load cell 328. Other sensors 330are also possible. The system could further include solid state relays364, a pneumatic cylinder 370 and an associated pneumatic control valve360 and solenoids 362. Additionally, the system could include ahydraulic cylinder 350 and associated hydraulic control valves 340,which could include, for example, a directional control valve 342, aflow control valve 344, and a lock valve 346.

The first actuator 108 and the second actuator 110 could take differentforms in various embodiments. For instance, as shown in FIG. 3, thefirst actuator 108 may be a hydraulic cylinder 350 configured to applyhydraulic forces to the first loading member 104 such that a tensileforce is applied to the tension member 102 in the first direction 112.In such implementations, the hydraulic cylinder 350 may include a powerunit (such as a pump, not shown). In other examples, the first actuatorcould be a load screw, a jack, and/or one or more counterweights.

Moreover, in some implementations, the second actuator 110 may be apneumatic cylinder 370 that is configured to push and/or pull the secondloading member 106 via pneumatic forces, such that the second loadingmember 106 is displaced along two or more directions 114 that aresubstantially transverse to the first direction 112. Specifically, thepneumatic cylinder 370 could be a single- or double-acting pneumaticcylinder that may utilize a compressed gas, such as air, to produce alinear force. Moreover, in such implementations, the pneumatic cylinder370 may by controlled by the pneumatic control valve 360. For example,the pneumatic control valve 360 could be a three-position, four-wayvalve that could be configured to be controlled by the control system300. The pneumatic control valve 360 could include solenoids 362 thatcould be configured to adjust the state of the pneumatic control valve360. For example, the pneumatic control valve 360 could be configured toextend and/or retract a rod of pneumatic cylinder 370. Additionally, thepneumatic valve 360 could be configured to operate in a vented state,where the pneumatic cylinder 370 may be substantiallyfree-moving/floating. In other examples, the second actuator 110 couldbe a voice coil/solenoid, a direct drive electric motor, and/or one ormore masses offset on a rotary motor (or located in-line with suitablehydraulics or pneumatics).

Further shown in FIG. 3, the control system 300 could include one ormore computers and/or one or more microcontrollers, such as an ArduinoMega 2560 R3 hardware board. In some examples, the microcontroller 302could be an 8-bit Atmel flash microcontroller or a 32-bit ARMinstruction-based processor. Other types of computers and/ormicrocontrollers are possible. The microcontroller 302 could include aUSB port and/or a RS-232 serial port. The microcontroller 302 could alsoinclude analog and/or digital input/output pins. The memory 304 couldrepresent internal memory, such as on-board flash memory. Alternativelyor additionally, the memory 304 could represent external memory, such asan SD card. The microcontroller 302 could additionally include a shieldthat could be configured to provide, for instance, WiFi/internetconnectivity. The shield could also be configured to provide an externalmemory interface, such as a micro SD slot. The control system 300 couldbe configured to perform various operations based on instructions 306that could reside, for instance, in memory 304. In some embodiments, acomputer-readable medium 308 could include various combinations ofmemory 304, instructions 306, and the microcontroller 302. The controlsystem 300 could include one or more solid state relays 364. The solidstate relays 364 could be configured, in response to a signal from themicrocontroller 302, to provide 120 VAC to the pneumatic control valve360. The pneumatic control valve 360 could be configured to controlvarious aspects of the second actuator 110. For example, in theembodiments where the second actuator 110 is a pneumatic cylinder 370, asolenoid 362 could change or maintain a given valve position of thepneumatic control valve 360 in response to the 120 VAC provided by thesolid state relays 364. In such a manner, the control system 300 couldbe configured to control the pneumatic control valve 360 and the forceapplied to a rod of the pneumatic cylinder 370. Accordingly, the controlsystem 300 could be configured to control the direction of the forceapplied to the second loading member 106 (pushing, pulling, ornone/freely-float). Accordingly, the control system 300 may beconfigured to indirectly affect the peak sinusoidal force in the tensionmember 102.

The control system 300 could also be configured to control the firstactuator 108. For example, the control system 300 could be configuredsuch that an analog or digital input to the control system 300 includesoutput from a pressure transducer 326 and/or the load cell 328. In suchan example, the microcontroller 302 could be configured to adjust theposition of a rod of the first actuator 108 (which could be a hydrauliccylinder) by controlling the hydraulic control valves 340 (e.g.,directional control valve 342, lock valve 346, flow control valve 344)in response to a signal from the pressure transducer 326 and/or the loadcell 328. Specifically, the control system 300 may be configured tocontrol the first actuator 108, such that the first loading member 104applies an initial base load on tension member 102. Alternatively oradditionally, the control system 300 could be configured to control thefirst actuator 108 such that the first loading member 104 applies atension force on tension member 102 based on, for instance, a desiredtension force range. In other examples, the force applied by the firstactuator 108 to the first loading member 104 may vary based on changesin one or more parameters of the frame 116 (e.g., expansion and/orcontraction of the frame 116 based on temperature loading).

The control system 300 could be additionally configured to control thesecond actuator 110. As described above, the second actuator 110 couldbe a pneumatic cylinder 370. In such an embodiment, in response to aninput from the second sensor 210, the control system could control asolid state relay 364 to supply 120 VAC to one or more solenoids 362associated with the pneumatic control valve 360. The 120 VAC signalcould cause the one or more solenoids 362 to change the currentconfiguration of pneumatic control valve 360. The behavior of thepneumatic cylinder 370 may be based on the current configuration ofpneumatic control valve 360. Accordingly, the control system 300 couldbe configured to control the pneumatic cylinder 370 to extend and/orretract a rod (or “freely-float”). Specifically, the control system 300could be configured to control the second actuator 110 such that theloading system enters a transverse mode resonance state at or near aresonance frequency of the loading system. The control system 300 couldbe further configured to control the second actuator 110 so as tocontrol the amplitude of vibration of the loading system such that thetension force in tension member 102 follows a desired cyclic loadingprofile. The desired cyclic loading profile could be based on a baseload and a peak load. The base load could be an initial base tensionapplied to the tension member 102. The peak load could be the tensionforce in the tension member 102 at a maximum displacement of the coupler208 during the vibration of the loading system. The control system 300could be configured to perform other functions.

The control system 300 could be configured to accept signals fromenvironmental sensors, such as humidity, altitude, and/or temperaturesensors. Correspondingly, the control system 300 could be configured toadjust various control parameters based on, for example, environmentalconditions.

The control system 300 could be configured to continuously orperiodically monitor and adjust the first actuator 108 and/or the secondactuator 110 so as to maintain the tension member 102 in a transversemode at or near a resonant frequency. For example, the control system300 could be further configured to determine the current tensile load onthe tension member 102 and maintain the tensile load within a desiredtensile load range. Additionally or alternatively, the control system300 could maintain the tensile load on tension member 102 based on thecycle load profile.

The first loading member 104 and the second loading member 106 couldtake various different forms in various different embodiments. In someimplementations, the first loading member 104 and the second loadingmember 106 may be one or more steel structures, such as beams. As shownin FIGS. 1 and 2, the second loading member 106 may have a dimensionthat is less than a respective dimension of the tension member 102. Forinstance, the second loading member 106 may have a length of 3 feet, andthe tension member 102 may have a length of 30 feet. In some examples,the second loading member 106 may include one or more bearings (e.g.,cylindrical or linear roller bearings, not shown) that may reduce energylosses during the oscillation of the second loading member 106, such asenergy losses due to friction.

In other examples, the second loading member 106 may be a combination ofone or more structures and cables, such as a four-bar (or cable)linkage. In such examples, the second loading member 106 may include oneor more cables configured to stabilize or reduce out-of-plane motion ofthe second loading member 106 during the oscillation of the secondloading member 106.

In other examples, the second loading member 106 may be similar oridentical to any of the tension members described herein. In suchexamples, one or more parameters of the tension member 102 and thesecond loading member 106 may be tested simultaneously.

As noted above, embodiments described herein may be used to teststiffness and/or strength of tension members (e.g., monotonic tensiletests). For instance, in some examples, the second loading member 106may be removed, and the tension member 102 may be directly connected tothe frame 116. Further, in such examples, the first sensor 206 and/orthe third sensor may sense information during the test. Other equipmentmay be used as well, including an extensomers, visual markers, and oneor more cameras.

The tension member 102 could include, for example, ahigh-tensile-strength core constructed of carbon fiber, steel, or othersuitable material. The core could be surrounded by strands of anelectrically-conductive material, such as aluminum alloy. The tensionmember 102 could also include instrumentation for measuring position,angle, and/or forces, such as tension. Such instrumentation could beincorporated into the tension member 102 itself, or could be mounted onthe tension member 102.

Note that, although the tension member 102 may be described as a tetherused as part of an AWT, the embodiments described herein may also beused to test tension members for use in other applications, and/or totest other types of tension members including ropes, cables, umbilicals,etc. used in variety of applications, such as overhead transmission,aerostats, offshore drilling, ROVs, towing, mining, and/or bridges,among other possibilities.

III. Illustrative Methods

A method 400 is provided for applying tension forces to a tension memberby, for example, causing the tension member to oscillate in a transversemode. The method could be performed using any of the apparatus shown inFIGS. 1-3 and described above. However, other configurations could beused. FIG. 4 illustrates the blocks of an example method with referenceto FIGS. 1-3. However, it should be understood that in otherembodiments, the blocks may appear in different order and blocks couldbe added or subtracted.

Block 402 includes translating the first loading member 104 in the firstdirection 112. In doing so, a tensile load could be applied to thetension member 102. In such a case, the first end of the tension member102 could be connected to the first loading member 104. The translatingof the first loading member 104 could be performed in various ways. Forexample, the control system 300 could control the first actuator 108 toapply a force to the first loading member 104 so as to apply a tensionforce to tension member 102. In some embodiments, the load applied totension member 102 could be measured by the load cell 328. Additionallyor alternatively, the load could be measured indirectly by the pressuretransducer 326. The control system 300 could control the first actuator108, for example, by adjusting the hydraulic control valves 340associated with the first actuator 108 in response to information fromthe load cell 328 and/or the pressure transducer 326. In otherembodiments, the first loading member 104 could be translated in thefirst direction 112 using other mechanical features and/or componentssuch as a load screw, a jack, and/or one or more counterweights.

Block 404 includes displacing the second loading member 106 along two ormore second directions 114 that are transverse to the first direction112 such that the tension member 102 vibrates at a frequency wherein thesecond end of the tension member 102 is connected to the second loadingmember 106. In some examples, the second actuator 110 may apply one ormore forces to the second loading member 106 at one or more forcingfrequencies to cause the second loading member 106 to vibrate. In suchimplementations, the one or more forcing frequencies may vary during theone or more oscillation cycles. The frequency at which the secondloading member 106 vibrates could include, for example, a resonantfrequency of a combination of loading system elements that could includethe tension member 102, the coupler 208, and/or other loading systemelements.

It may be beneficial for the forces applied to the second loading member106 to be substantially in phase with the vibration of the tensionmember 102. In some examples, a resonant frequency of the combination ofloading system elements may vary during the oscillation of the secondloading member 106 based, at least in part, on a change in a stiffnessof the loading system in the transverse direction. Such a change instiffness may be due to an amount of deflection of the tension member102. Accordingly, the control system 300 may control the second actuator110 to (i) cause the tension member 102 to vibrate at a resonantfrequency and (ii) maintain the vibration by controlling an amplitude ofthe vibration.

In some examples, the resonant frequency of the combination of loadingsystem elements may not be known before cyclic displacement of thesecond loading member 106. As a result, one or more forcing frequenciesmay be selected to excite the vibration of the tension member 102. Forinstance, in some implementations, the one or more forcing frequenciesmay be selected based on a frequency sweep mode of the control system300. In the frequency sweep mode, the control system 300 may (i) selecta first frequency of the one or more forcing frequencies that is knownor believed to be less than the resonant frequency of the combination ofloading system elements; and (ii) increase the first frequency of theone or more forcing frequencies until the tension member 102 vibrates ata resonant frequency of the combination of loading system elements. Suchan approach may involve the second actuator 110 applying one or moreforces to the second loading member 106 that is less than respectiveforces applied in other modes, such as a step response mode, that couldbe used to cause vibration of the tension member 102 at a resonantfrequency of the combination of loading system elements.

After the tension member 102 is vibrating at the resonant frequency, thecontrol system 300 may control the amplitude of vibration of the tensionmember 102 by changing a direction of a force applied by the secondactuator 110 to the second loading member 106. Further, in suchimplementations, the control system 300 may change the direction of theforce applied to the second loading member 106 at one or more peakdisplacement locations of the tension member 102. For instance, when thesecond loading member 106 is configured to move back and forthsubstantially perpendicular to the ground, the control system 300 may(i) cause the second actuator 110 to apply an upward force to the secondloading member 106 until the second loading member 106 translates to amaximum point in the oscillation; (ii) cause the second actuator 110 toapply a downward force to the second loading member 106 until the secondloading member 106 translates to a minimum point in the oscillation; andthen repeat step (i). Additionally, the control system 300 may cause thesecond actuator 110 to change the magnitude of one or more forcesapplied to the second loading member 106 during the displacement of thesecond loading member 106.

In other implementations, the control system 300 may control theamplitude of vibration of the tension member 102 by causing the secondactuator 110 to apply a force to the second loading member 106 during aportion of a single oscillation cycle (which may be referred to asintra-cycle duty cycle). For instance, the control system 300 may causethe second actuator 110 to apply a force to the second loading member106 during one quarter of an oscillation cycle and allow the secondactuator 110 to “freely float” during the rest of the oscillation cycle.Other intra-cycle duty cycle control implementations are possible.

In other implementations, the control system 300 may control theamplitude of vibration of the tension member 102 by causing the secondactuator 110 to apply a force to the second loading member 106 duringsome, but not every oscillation cycle (which may be referred to asinter-cycle duty cycle). For instance, when a vibration of the secondloading member 106 includes four oscillation cycles, the control system300 may cause the second actuator 110 to apply a force to the secondloading member 106 during three oscillation cycles of the vibration, butnot during the fourth oscillation cycle. Other implementations of theinter-cycle duty cycle control method are possible so as to maintain thetension member 102 vibrating in a transverse mode at a resonantfrequency of a combination of loading system elements.

In an illustrative configuration, the control system 300 may control theamplitude of vibration of the tension member 102 by any combination ofthe techniques described above, including causing the second actuator110 to change a magnitude of the force, apply the force during a portionof an oscillation cycle, and/or apply the force during multipleoscillation cycles of the vibration.

Furthermore, an illustrative method could include determining a tensionload on the tension member 102, determining a velocity of the secondactuator 110, and controlling the second actuator 110 such that thetension load is maintained based on a desired base tension load and adesired peak tension load. The desired base tension load could be, forexample, the initial base tension applied by displacement of the firstloading member 104. The desired peak tension load could be, for example,a desired, known, or anticipated maximum load on tension member 102.Other desired base tension and desired peak tension values are possible.

IV. Conclusion

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

We claim:
 1. A system comprising: a tension member having a first endand a second end, wherein the first end of the tension member isconnected to a first loading member and the second end of the tensionmember is connected to a second loading member; a load cell; a firstactuator; a second actuator; and a control system comprising a processorand a memory, wherein the processor is configured to executeinstructions stored in the memory so as to carry out operations, whereinthe operations comprise: controlling the first actuator to displace thefirst loading member based on information received from the load cellsuch that a tensile load is applied to the tension member along a firstdirection; and controlling the second actuator to displace the secondloading member based on a desired cyclic loading profile such that thesecond loading member oscillates between two or more second directions,wherein the oscillation of the second loading member causes the tensionmember to vibrate at a frequency, and wherein the desired cyclic loadingprofile comprises a base load and a peak load.
 2. The system of claim 1,wherein controlling the first actuator is further based on a desiredbase tensile load.
 3. The system of claim 1, wherein the desired cyclicloading profile further comprises a desired oscillation frequency, andwherein controlling the second actuator is further based on the desiredoscillation frequency.
 4. The system of claim 1, wherein the operationsfurther comprise controlling the second actuator to control an amplitudeof a vibration of the tension member.
 5. The system of claim 1, whereincontrolling the second actuator causes the tension member to beperiodically loaded substantially in tension according to the desiredcyclic loading profile.
 6. The system of claim 1, wherein the firstactuator comprises a hydraulic cylinder, and wherein the second actuatorcomprises a pneumatic cylinder.
 7. The system of claim 1, furthercomprising a frame; wherein a first end of the first loading member isconnected to the tension member and a second end of the first loadingmember is connected to the frame; wherein a first end of the secondloading member is connected to the tension member and a second end ofthe second loading member is connected to the frame; wherein the frameis configured to resist one or more compressive stresses based on anarrangement of the first loading member, the second loading member, andthe tension member.
 8. The system of claim 1, further comprising atleast one sensor, wherein the operations further comprise receivinginformation indicative of a position of the second loading member, andwherein controlling the second actuator is further based on the positionof the second loading member.
 9. The system of claim 1, furthercomprising at least one sensor, wherein the operations further comprisereceiving information indicative of a velocity of the second loadingmember, and wherein controlling the second actuator is further based onthe velocity of the second loading member.
 10. The system of claim 1,wherein controlling the second actuator comprises: while driving aposition of the second actuator according to a forcing frequency,adjusting the forcing frequency within a frequency range; determining aresonant frequency of the tension member based on at least one of: thefrequency of vibration of the tension member or an amplitude of thetension member; and driving the position of the second actuatoraccording to the resonant frequency.
 11. A method comprising:controlling a first actuator to displace a first loading member based oninformation received from a load cell such that a tensile load isapplied to a tension member along a first direction, wherein the tensionmember has a first end and a second end, wherein the first end of thetension member is connected to the first loading member and the secondend of the tension member is connected to a second loading member; andcontrolling a second actuator to displace the second loading memberbased on a desired cyclic loading profile such that the second loadingmember oscillates between two or more second directions, wherein theoscillation of the second loading member causes the tension member tovibrate at a frequency, and wherein the desired cyclic loading profilecomprises a base load and a peak load.
 12. The method of claim 11,wherein controlling the first actuator is further based on a desiredbase tensile load.
 13. The method of claim 11, wherein the desiredcyclic loading profile further comprises a desired oscillationfrequency, and wherein controlling the second actuator is further basedon the desired oscillation frequency.
 14. The method of claim 11,further comprising controlling the second actuator to control anamplitude of a vibration of the tension member.
 15. The method of claim11, wherein controlling the second actuator causes the tension member tobe periodically loaded substantially in tension according to the desiredcyclic loading profile.
 16. The method of claim 11, wherein the firstactuator comprises a hydraulic cylinder, and wherein the second actuatorcomprises a pneumatic cylinder.
 17. The method of claim 11, furthercomprising receiving, via at least one sensor, information indicative ofa position of the second loading member, and wherein controlling thesecond actuator is further based on the position of the second loadingmember.
 18. The method of claim 11, further comprising receiving, via atleast one sensor, information indicative of a velocity of the secondloading member, and wherein controlling the second actuator is furtherbased on the velocity of the second loading member.
 19. The method ofclaim 11, wherein controlling the second actuator comprises maintainingthe base load and the peak load according to the desired cyclic loadingprofile.
 20. The method of claim 11, wherein controlling the secondactuator comprises: while driving a position of the second actuatoraccording to a forcing frequency, adjusting the forcing frequency withina frequency range; determining a resonant frequency of the tensionmember based on at least one of: the frequency of vibration of thetension member or an amplitude of the tension member; and driving theposition of the second actuator according to the resonant frequency.